The present invention relates to an image display technique, and particularly to a rear projection screen which is suitably used as a screen for a projection television, a microfilm reader or the like. The rear projection screen of the present invention is suitably applicable to a rear projection screen onto which an optical image formed on a light valve (light source) having matrix-arranged picture element portions (that is, having such a structure that picture element portions are arranged in a matrix form) such as an LCD (liquid crystal device) projector or DMD (digital micromirror device) projector is projected.
For rear projection type televisions, it has been hitherto required that projected images can be brightly viewed in a wide viewing angle range at the viewing side, and rear projection screens having anisotropy in visual field which are designed to diffuse light broadly particularly in a horizontal direction and diffuse light properly in a vertical direction although the diffusion angle range in the vertical direction is smaller than that in the horizontal direction have been used.
A lenticular lens sheet is known as one of these rear projection screens. In the lenticular lens sheet, lenticular lenses each extending in the vertical direction are arranged in parallel on one surface or both surfaces of a sheet to provide the sheet with a light diffusion property to form a light diffusion sheet, and light diffusing members or light diffuser are contained in the diffusion sheet so that light is broadly diffused in the horizontal direction by the lenticular lenses and also it is diffused to some extent in the vertical direction by the light diffuser.
Besides, a projector using a device for performing a display operation by using a matrix pixel structure such as LCD or DMD in place of CRT has propagated as a projection image source used in combination with the rear projection screen. Such a projectior structurally suffers no geomagnetic effect unlike a CRT projector, and thus it is very preferably used as an image light source for a display device of a computer such as a personal computer or the like which is usually used to display still images. Such a rear projection screen using LCD or DMD as a projector has been required to have new performance because it is also used for a display device having a relatively small area of about 14 to 40 inches, such as a personal computer monitor for viewing images at a relatively near position to the screen of the device.
That is, it has been required (1) to cancel a moire phenomenon or scintillation phenomenon occurring due to the interference between periodical structures of projection pixels and lenticular lenses, (2) to cancel a speckle phenomenon occurring due to the interference between projection light and light diffuser added in the lenticular lenses or due to the glare of minute projections and recesses on the surface of the screen or the diffuser (hereinafter referred to as xe2x80x9cspecklexe2x80x9d), and (3) to clearly resolve images of conventional VGA, SVGA and further high pixel number graphics of XGA, SXGA, UXGA, etc.
For these performance requirements, the following resolving proposals have been made for not only a rear projection screen for a projector using LCD or DMD, but also a rear projection screen for a projector using CRT or the like.
With respect to (1), each of Japanese Patent Application Laid-open Publication No. Hei-3-168630 and Japanese Patent Application Publication No. Hei-7-117818 proposes a method of canceling the moire phenomenon by optimizing the pitch ratio between the projection pixels and the lenticular lenses, and each of Japanese Patent Application Laid-open Publication No. Hei-2-123342 and Japanese Patent Application Laid-open Publication No. Hei-2-212880 proposes a method of canceling the moire phenomenon by inclining the lenticular lenses relatively to the projection pixels.
As described above, the moire phenomenon occurring due to the periodical structure of the lenticular lenses and the projection pixel pitch can be canceled by optimizing the pitches thereof. However, when the number of pixels is increased to the level of XGA class or SXGA class or more or when an image is projected onto a relatively small screen of about 14 to 40 inches, the pitch of the lenticular lenses must be reduced to a very small value of about 0.1 mm or less to cancel the moire phenomenon because the pitch of the pixels constituting an image projected on the rear projection screen is very small, and thus there are problems that it is very difficult to manufacture a mold for such lenses, it is impossible to transfer the lens shape accurately, and the lifetime of the mold is reduced.
With respect to (2), each of Japanese Patent Application Laid-open Publication No. Hei-8-313865, U.S. Pat. No. 5,675,435, U.S. Pat. No. 3,712,707 and Japanese Patent Application Laid-open Publication No. Sho-55-12980 proposes a method of reducing the speckle by dividing a light diffusion layer or grading the concentration of the light diffuser in the thickness direction.
With respect to (3), Japanese Patent Application Laid-open Publication No. Sho-55-12980 discloses that the thickness of the diffusion layer is reduced to 100 xcexcm or less to obtain a rear projection screen having resolving power higher than that of the human eyes (5 to 10 lines/mm).
However, any of the above prior arts cannot satisfy all of the performance requirements (1) to (3). Particularly, the reduction of the speckle in (2) and the high resolving power in (3) are in tradeoff relationship with each other. If the reduction of the speckle is attempted, the resolving power is reduced. If the resolving power is increased, the speckle is more remarkable. For example, in Japanese Patent Application Laid-open Publication No. Hei-8-313865, the speckle can be reduced by dividing a light diffusion layer and setting the distance from the light incident face of a first light diffusion layer to the light emission face of a second light diffusion layer to 1.5 mm or more. However, in the case of the high pixel number such as XGA, SXGA class or higher case, the resolving power is reduced and thus a projected image of high resolution cannot be provided. Further, if the thickness of the diffusion layer is reduced to 100 xcexcm or less as in the case of Japanese Patent Application Laid-open Publication No. Sho-55-12980, occurrence of speckle is remarkable although a projected image of high resolution is obtained, and thus a high quality projection cannot be provided.
Further, a sheet which is formed of methacrylic resin, polycarbonate resin or the like and contains an inorganic or organic light diffuser is generally used as a light diffusion sheet used for such a rear projection screen or the like.
The light diffuser is required to have the following characteristics: it enhances the light diffusion performance of the light diffusion sheet and the rear projection screen and has high total-light transmittance and a high light using efficiency; it has proper color temperature; and it exhibits neither the see-through feature that a light source image such as a lamp image, CRT or liquid crystal projector is seen through the screen nor the hot band that striped bright portions are partially viewed.
As the light diffuser having the above characteristics are used inorganic light diffuser such as silica, muscovite, alumina, calcium carbonate, glass beads or the like as disclosed in Japanese Patent Application Laid-open Publication No. Sho-60-46503, or resin beads of acrylic resin or styrene resin as disclosed in Japanese Patent Application Laid-open Publication No. Sho-61-4762.
In order to keep the light transmission and the light diffusion in balance, there have been proposed various light diffusion sheets and various compositions using silicone-based light diffuser. Examples thereof are as follows: a light diffusion plate composed of transparent resin dispersed with light diffuser which is made of spherical, solid silicone resin, has a size of 0.3 to 10 xcexcm and has polysiloxane coupling, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-1-172801; a light diffusing synthetic resin composed of transparent synthetic resin dispersed with silicone resin fine particles of 1 to 6 xcexcm and inorganic transparent powder, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-2-194058; a molded product composed of methacrylic resin and polymethylsilsesquioxane particles of 0.5 to 20 xcexcm, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-3-207743; a light diffusing resin composition composed of methacrylic resin and spherical silicone resin having phenyl groups, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-5-39401; a light diffusing resin composed of methacrylic resin, crosslinked methacrylic resin fine particles and silicone fine particles of 1 to 20 xcexcm, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-6-107881; a light diffusing resin composition composed of polycarbonate and polymethylsilsesquioxane particles, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-6-192556; a light diffusing resin composition composed of polymer mainly containing methylmethacrylate dispersed with liquid polysiloxane, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-7-207101; a light diffusion resin composition composed of acrylic resin and silicone rubber powder, as disclosed in Japanese Patent Application Laid-open Publication No. Hei-10-87941.
However, the light diffusing sheet containing inorganic light diffuser such as glass beads disclosed in Japanese Patent Application Laid-open Publication No. Sho-60-46503 and the light diffusing sheet containing light diffuser composed of silicon resin having high hardness as disclosed in Japanese Patent Application Laid-open Publication No. Hei-1-172801 and so on have such a problem that the strength of the transparent resin serving as a base member is lowered. Particularly when they are used as a light diffusion layer of a rear projection screen to which high resolution is required, the thickness of the light diffusion layer must be reduced to 1.5 mm or less, and it is necessary to increase the content of light diffuser in order to obtain a wide visual field as the thickness of the light diffusion layer is reduced. However, since such a light diffusion layer has a small thickness and contains a large amount of light diffuser, it is very difficult to manufacture the light diffusion layer, and also it is very difficult or cumbersome to handle the diffusion layer when it is carried or fabricated. In addition, it is easily cracked due to an external shock even after it is set up.
When a light diffusing sheet is formed by an extrusion method using resin containing light diffuser, there are problems that the resin is liable to be accumulated at a portion (lip portion) through which the resin is jetted from a die, the light diffusing sheet comes into contact with the portion at which the resin is accumulated, thereby easily inducing appearance defect, and it is difficult to manufacture the light diffusing sheets stably for a long term.
Further, the degradation of the shock resistance of the light diffusion sheet and the occurrence of the appearance defect can be suppressed to some extent by using the rubber silicone light diffuser as disclosed in Japanese Patent Application Laid-open Publication No. Hei-10-87941. However, when the rubber light diffuser is used, agglomeration of rubber particles is remarkable and thus dispersibility into the base resin is degraded.
Therefore, the present invention has an object to provide a rear projection screen which can obtain a high-quality projected image with high resolution while moire and speckle occur hardly when being used in combination with a liquid crystal projector, etc.
Further, the present invention has another object to provide a rear projection screen which has a sufficiently wide angle of visual field and is excellent in shock resistance and light transmission.
That is, according to a first aspect of the present invention, a rear projection screen onto which an optical image formed on a light valve having picture element portions arranged in a matrix form is projected, is characterized by comprising at least a first light diffusion layer and a second light diffusion layer, wherein the first light diffusion layer comprises a first base material formed of transparent resin and a first light diffuser formed of transparent fine particles contained in the first base material by 20 to 50 wt %, the difference in refractive index between the first base material and the first light diffuser being equal to 0.07 to 0.17, the weight average particle diameter of the first light diffuser being equal to 1 to 12 xcexcm, the thickness of the first light diffusion layer being set to 50 to 200 xcexcm, and the second light diffusion layer comprises a second base material formed of transparent resin and a second light diffuser which is formed of transparent fine particles having a weight average particle diameter of 1 to 12 xcexcm, the thickness of the second light diffusion layer being set to 500 to 5000 xcexcm and the haze value of the second light diffusion layer being set to 50 to 85%.
According to a second aspect of the present invention, a rear projection screen onto which an optical image formed on a light valve having picture element portions arranged in a matrix form is projected, is characterized by comprising at least a first light diffusion layer and a second light diffusion layer, wherein the first light diffusion layer comprises a first base material formed of transparent resin and a first light diffuser formed of transparent fine particles contained in the first base material by 20 to 50 wt %, the difference in refractive index between the first base material and the first light diffuser being equal to 0.07 to 0.17, the weight average particle diameter of the first light diffuser being equal to 1 to 12 xcexcm, the thickness of the first light diffusion layer being set to 50 to 200 xcexcm, and the second light diffusion layer comprises a second base material formed of transparent resin and a second light diffuser which is contained in the second base material by 0.1 to 10.0 wt % and formed of transparent fine particles having a weight average particle diameter of 1 to 12 xcexcm, the thickness of the second light diffusion layer being set to 500 to 5000 xcexcm.
According to a third aspect of the present invention, a rear projection screen onto which an optical image is projected by projected light, is characterized in that a polarizing film layer having a polarization degree of 96% or more is disposed between a first light diffusion layer and a second light diffusion layer, and the first light diffusion layer, the polarizing film layer and the second light diffusion layer are laminated.
According to a fourth aspect of the present invention, a rear projection screen onto which an optical image formed on a light valve having picture element portions arranged in a matrix form is projected, is characterized by including: a light diffusion layer of 0.3 to 1.2 mm in thickness which comprises a transparent resin and light diffuser having a refractive index different from that of the transparent resin by 0.05 or more and a volume average particle diameter of 1 to 8 xcexcm, the light diffuser being dispersed at a concentration of 10 g/m2 to 60 g/m2 in the transparent resin; and a polarizing film layer.
According to a fifth aspect of the present invention, a rear projection screen onto which an optical image is projected by projected light, is characterized in that a polarizing film layer having a polarizing degree of 96% or more is disposed between a light diffusion layer and a transparent plastic layer, and the light diffusion layer, the transparent plastic layer and the polarizing film layer are laminated.
A rear projection screen according to a sixth aspect of the present invention, is characterized by including a light diffusing sheet in which light diffusing coated particles of a volume average particle diameter of 1 to 8 xcexcm having a resin layer on the surface of each of silicone rubber spherical fine particles are contained at a concentration of 0.01 to 100 g/m2 in a transparent resin having a refractive index different from that of the silicone rubber spherical fine particles by 0.06 or more.